This invention relates to the preparation of biodegradable thermoplastic graft copolymers. More particularly, this invention relates to an improved process for converting a polysaccharide to a biodegradable thermoplastic graft copolymer not only capable of forming homogeneous blends with synthetic thermoplastic polymers itself, but further capable of functioning as compatibilizing/stabilizing agents for blends of cellulose and starch with synthetic plastics. Nucleophilically displaceable groups are formed on the polysaccharide and are thereafter displaced by selected polymers having anionic carboxylate or thiocarboxylate groups, to provide high yields of well-defined, multifunctional biodegradable thermoplastic graft copolymers.
Increasingly strident environmental concerns have put pressure on federal and state legislatures to mandate plastics degradability. This is a push that directly affects an annual 1.8 billion pounds of business, mostly packaging, done by the plastics industry. Plastics take as much as 200 years to degrade in landfill. Plastic litter dumped into the oceans every day cause heavy loss of marine and animal life. Naturally occurring biopolymers like starch and cellulose are readily biodegradable (degrades only in soil, sewage and marine environments where bacteria are active--that is, biologically active environments only--precisely the conditions where onset of degradation is desired). Incorporation of these types of biopolymers plastics (styrenic plastics or polyethylene or polymethyl methacrylate plastics) by blending or graft copolymerization should lead to a new type of plastics having the trait of biodegradability. However, preparing a new material system by mixing two incompatible polymers as in the present case results in products with reduced physical properties. Strength and toughness values are minimal and are lower for the mixture than any of the pure components. This situation arises from poor interfacial adhesion between the individual components due to their inherent incompatibility. It is like trying to mix or disperse oil and water. The solution to this incompatibility problem which is widely practiced in the polymer industry, uses block or graft copolymers of the form A-B as compatibilizers or interfacial agents to improve adhesion between immiscible A rich and B-rich phases. To function effectively as a compatibilizer, the following is true: (1) components of the graft copolymers must be identical with the polymers in the two phases (identical with the 2 dissimilar polymers which needs to be blended); (2) molecular weight of the segments plays an important role, and control over the molecular weights is essential; (3) molecular weights greater than 150,000 is generally a poor compatibilizer; (4) block or graft copolymers segments containing 10-15 monomer units is an effective compatibilizing agent for the corresponding higher molecular weight homopolymer.
Thus, the key to the incorporation of natural biopolymers like starch and cellulose in a plastics materials system to make biodegradable/bio based plastics is the ability to tailor cellulose/starch synthetic polymer graft copolymer structures with control over the molecular weights of the graft, degree (amount) of graft substitution and control over backbone graft linkage. Current technology does not permit the making of cellulose/starch (natural biopolymers)--synthetic polymer graft copolymers with precise control over molecular weights, degree of substitution, backbone graft linkage, etc., i.e., cannot make precise tailor made cellulose/starch graft copolymers.
The present invention allows the preparation of tailor-made cellulose/starch synthetic polymer graft copolymers with good control over molecular weights degree of substitution, backbone-graft linkage. These graft copolymers can function effectively as compatibilizing agents/interfacial agents for compounding/blending of cellulose and starch with synthetic polymers. The graft copolymer allows a fine dispersion of the natural polymer into the plastic phase without detracting from the excellent mechanical and thermal properties inherent in the plastic, while incorporating a new trait of biodegradability.
The grafting of synthetic polymers onto polysaccharides and polysaccharide derivatives has been described in the art. Preparation of cellulosic graft polymers utilizing free radical polyerization methods has been reported by McDowall, Gupta, and Stannett, Prog. Polym. Sci. 1984, 10, 1; Hebeish and Guthrie, The Chemistry and Technology of Cellulosic Copolymers, Berlin, 1981; Arthur, Adv. Macromol. Chem. 1970, 2, 1 See also U.S. Pat. No. 4,026,849. Polyisobutylene-grafted cellulose products have been prepared by reacting anhydride-terminated polyisobutylene with sodium cellulosate. Coleman-Kammula and Hulskers Wood and Cellulosics--Industrial Utilisation, Biotechnology, Structure and Properties 1987, 195-202. The successful grafting of the polyisobutylene onto cellulose involves the conversion or activation of cellulose to cellulosates. Polystyrene has been grafted onto cellulose acetate with a grafting yield of up to 83% using the acid chloride of carboxylic acid-terminated polystyrene. Mansson and Westfelt, J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 1509. This method involves the acylation of the free hydroxyl groups on the cellulose acetate by the polystyrene acid chloride. Other known methods for preparing graft copolymers include the simultaneous polymerization and grafting of an ethylenically unsaturated monomer onto the molecule of a polysaccharide and thereafter reacting the grafted polysaccharide, in the presence of a catalyst, with an acylating agent to form a polysaccharide ester derivative. See U.S. Pat. No. 3,332,897.
The known methods for synthesizing polysaccharide-synthetic polymer graft copolymers have several disadvantages. For example, the molecular weights of graft copolymers produced by free radical polymerization techniques are very high and the molecular weight distribution in such copolymers is polydisperse. The reproducibility of these polymerization methods is also poor and there is little control over the grafting process. Thus, the resultant graft copolymers exhibit a low level of graft substitution typically with very high molecular weight graft molecules. Likewise, products prepared by other prior art polymerization reactions have considerable disadvantages. For example, products prepared by reacting a polysaccharide and an acid anhydride cannot be molded easily, if at all. Typically, such molded products are brittle, inflexible and entirely unsuitable for commercial utilization. Moreover, to achieve high grafting efficiencies, strictly anhydrous conditions must be used along with fairly large amounts of acylation catalysts such as 4-(dimethylamino) pyridine with reaction periods of up to 3 days. Thus, there is the need in the art for an improved method of synthesizing biodegradable polysaccharide graft copolymers.
It is an object of this invention to provide an improved method of preparing polysaccharide-synthetic polymer graft copolymers.
A further object of this invention is to provide an economic, commercially feasible procedure for achieving high yields of well-defined biodegradable graft copolymers.
An additional object of this invention is to provide a grafting method which allows greater control of the molecular weight distribution and the number and nature of the grafted sidechains.
Still another object of this invention is the use of such novel biodegradable thermoplastic copolymers alone and for blending with synthetic thermoplastic polymers, with or without other added biodegradable polysaccharides, to provide homogeneous, tough, high strength biodegradable plastics.